Losing detail when retopologizing
Use the shrinkwrap-modifier in combination with a subsurf- or multires-modifier.
Make sure that the shrinkwrap is stacked lower than the subsurf- or multires-modifier to gain detail.

When using the multires-modifier, you can apply the shrinkwrap-modifier and the gained detail will be 'absorbed' by the multires. You can then enter sculpt mode to add and or smooth the desired detail while keeping your relatively low poly retopologized mesh.